1. Field of the Invention
The present invention relates to a piezoelectric filter of an energy-trapped type using a second harmonic wave of the thickness-extensional vibration mode.
2. Description of Related Art
FIGS. 8 and 9 show a conventional piezoelectric filter of an energy-trapped type using a second harmonic wave of the thickness-extensional vibration mode.
As shown in FIG. 8, in a piezoelectric filter 1, an earth electrode 3 is arranged in the center, in the direction of thickness, of a piezoelectric substrate 2, which is polarized in the direction of thickness as indicated by the arrows. On the upper surface 2a of piezoelectric substrate 2, an input electrode 4 is formed so as to be opposed to a part of the earth electrode 3 through a piezoelectric substrate layer. On the other hand, an output electrode 5 is formed on the lower surface 2b of piezoelectric substrate 2. The output electrode 5 is formed so as to be opposed to the other part of the earth electrode 3 through a piezoelectric substrate layer, and is positioned so as not to overlap with the input electrode 4 through the piezoelectric substrate 2.
FIG. 9 is an exploded perspective view showing plane shapes and relative locational positions of the aforementioned earth electrode 3, input electrode 4, and output electrode 5. In FIG. 9, the output electrode 5 is shown by the lower surface 2b of the piezoelectric being projected below.
The aforementioned conventional piezoelectric filter 1 is a double-mode filter using a second harmonic wave of the thickness-extensional vibration mode and is intended to be used in a high-frequency region.
Although the piezoelectric filter 1 provides the energy-trapped type of piezoelectric filter operable in a high-frequency region, its insertion loss is relatively large and, therefore, a reduction of such insertion loss is desirable.
Furthermore, in the aforementioned piezoelectric filter 1, a reduction of matching impedance also has been desired so that the piezoelectric filter has application to wider uses. In order to reduce the matching impedance, it has been considered to increase the area of the electrodes in the piezoelectric filter 1, that is, the area of the earth electrode 3 and the input and the output electrodes 4 and 5. However, an increase in the electrode area leads to the reduction of matching impedance, accompanied by a problem of deterioration of the characteristics, which is caused by the enhancement of the level of unwanted spurious vibration.